Anti-stall motor drive

ABSTRACT

An anti-stall motor drive circuit for a very slow running permanent magnet d.c. motor is disclosed. The slow running speed of the motor sometimes allows an insulating film to build up between the brushes and the commutator which causes the stalled condition. The motor (10) is connected in the collector circuit of a drive transistor (Q1) which has a current limiting resistor (R9) connected in its emitter circuit. A stalled condition of the motor (10) is sensed by a lack of output from a tachometer (16) output. The output of the tachometer is full wave rectified by an operational amplifier (22) and diodes (CR1 and CR2). The rectified tachometer voltage is applied to an integrator (24 and C1) having a clamping diode (CR7) connected in its feedback circuit. A zero output from the rectifier allows the integrator output to ramp up to a limiting value determined by the clamping diode. The output of the integrator is applied to the base of the drive transistor to cause a voltage approximately fifteen times that of normal operating voltage to be impressed across the motor. This &#34;jolt&#34;, which is twice the rated voltage of the motor, overcomes the stalled condition and burns off any insulating film which may have developed between the commutator and the brushes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to improvements in electricmotor drive circuits and, more particularly, to an anti-stall drivecircuit for a permanent magnet d.c. motor which is operated at very slowspeed. The invention has particular application in railway datarecorders of the type that use a closed-loop magnetic tape data pack.

2. Description of the Prior Art

Railway recorders are installed on locomotives to record variousparameters of operation including speed, elapsed time, brake applicationand the like. These recorders are analogous to flight recorders oncommercial aircraft and allow for analysis of the events which occuredprior to an accident. In addition, the recorders may be used to verifyrules compliance by monitoring daily operations over a specific districtor on a system wide basis.

Early railway recorders used chart paper and included one or more penstyli to make the recording on the chart paper. These early recordershave now been generally replaced by magnetic tape recorders which useclosed-loop magnetic tape data packs. The data packs, which are similarto the familiar audio eight-track tape cartridges, eliminate the needfor frequent reloads and the risk of running out of paper. And becauseof their cartridge-type packaging, the data packs can be easily removedand replaced by unskilled railway personnel. The recorded data may be"readout" and plotted on chart paper specially designed for the purposeby means of a portable playback unit simply by removing the data packfrom the recorder and inserting it into the playback unit.

Railway recorders and playback units of the type described aremanufactured and sold by Pulse Electronics, Inc., of Rockville,Maryland. A specific example of such equipment is the 48H SeriesLocomotive Recording System. The recorder in this system uses apermanent magnet d.c. motor to drive the capstan and is characterized bya very slow capstan speed. The slow capstan speed, and hence tape speed,is justified by the fact that events are being recorded, and therefore awide frequency bandwidth is not required, and it is desirable tomaximize the elapsed time that can be recorded on the tape loop of thedata pack. The slow speed of the capstan is accomplished in part by areduction gear train, but this alone was found not to produce a slowenough tape speed. Therefore, the motor drive circuit was speciallydesigned with tachometer feedback to produce the desired slow tapespeed.

The problem with a very slow operation of a permanent magnet d.c. motoris that the motor appears to develop a high impedance contaminationbetween the commutator and the brushes. At higher speeds, suchcontamination does not develop presumably because the rotor turns fastenough to either prevent the contamination from developing or to cleanthe contamination off the commutator as it forms. Unfortunately, no suchself-cleaning action occurs on the commutator of a permanent magnet d.c.motor operated at very slow speeds in the locomotive recording system.As a result, there is occassionally experienced a stoppage of the motordue to a lack of electrical conduction between the commutator and thebrushes. This is easily remedied by spinning the rotor of the motor, butin the meantime valuable data may have been lost.

What was needed was a way in which the stopped condition of the motorcould be automatically detected and the motor restarted so that therewould be no loss or only a minimal loss of data. The problem appears tobe unique to the very slow speed operation of permanent magnet d.c.motors, and no solution was known in the prior art. Moreover, a.c. motorcontrol circuits of the type which sense motor torque to develop acontrol signal such as shown in U.S. Pat. No. 3,519,910 to Pfaff et al.are not directly applicable to this particular problem. Other tape speedcontrol circuits using a.c. motors such as shown in U.S. Pat. No.3,634,744 to Toensing et al. and U.S. Pat. No. 3,800,200 to Bunting failto suggest any remedy to the problem.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved motorcontrol circuit which prevents stalling of permanent magnet d.c. motorswhich are operated at very low speeds.

It is another object of the invention to provide an anti-stall motordrive circuit for permanent magnet d.c. motors which adds little to theexpense of manufacture of the drive circuit.

It is a further object of this invention to provide a simplemodification to an existing motor drive circuit for railroad locomotivedata recorders which allows for retrofit of existing data recorders.

According to the invention, a stalled condition of the motor is sensedby a lack of output from the tachometer feedback. This condition allowsan operational amplifier with capacitive feedback to provide an outputwhich ramps up to a limiting level. The operational amplifier outputsupplies an increasing drive voltage to the base of the motor drivetransistor to cause a voltage approximately fifteen times that of normaloperating voltage to be impressed across the motor. This "jolt", whichis twice the rated voltage of the motor, overcomes the stalled conditionand burns off any high impedance contamination which may have developedbetween the commutator and the brushes.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other objects, aspects and advantages of the inventionwill be better understood from the following detailed description of theinvention with reference to the drawing in which the sole figure is aschematic diagram of the improved motor drive circuit according to thepreferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing, the permanent magent d.c. motor 10 isconnected in the collector circuit of an NPN transistor Q1. Morespecifically, motor 10 is connected between a pair of terminals 12 and14. Terminal 12 is connected via a current limiting resistor R8 and adiode CR5 to the +74 volt battery positive (BP) from the locomotive cabelectrical locker. The +74 volt battery voltage is smoothed andregulated by a capacitor C2 and a Zener diode CR6 connected in parallelwith the locomotive battery supply. Terminal 14 is directly connected tothe collector of transistor Q1. The emitter of transistor Q1 isconnected through resistor R9 to provide a return to battery negative(BN) which is -11 volts, and a biasing resistor R7 is connected betweenthe emitter and the base of transistor Q1.

Transistor Q1 provides the basic drive for the motor 10 by controllingthe current flowing through the motor. In the prior recorder circuit,the motor was connected in the emitter circuit of transistor Q1. Movingthe motor to the collector circuit of transistor Q1 and adding theemitter resistor R9 is one of the modifications made to the earliercircuit by this invention.

The base drive for transistor Q1 is developed by the output of atachometer 16 which is driven by the motor 10. Tachometer 16 isconnected to a pair of terminals 18 and 20 and provides a sinusoidalvoltage output. Resistors R1 and R4 are precision resistors which, incombination with potentiometer R70, allow for precise calibration of thevoltage applied across the tachometer 16. Terminal 18 is also connectedto the inverting or negative input terminal of operational amplifier 2via a precision input resistor R2, while terminal 20 is connected incommon with the noninverting or positive input terminal of operationalamplifier 22 to circuit ground.

Amplifier 22 is connected as a full wave rectifier to provide arecitified, positive d.c. voltage output which is proportional to thespeed of the tachometer 16 and, hence, the speed of motor 10. A diodeCR1 is connected between the inverting input of operational amplifier 22and its output and poled to allow the passage of current only from theinverting input to the output of the amplifier. Thus, diode CR1 servesto bypass the amplifier during positive half cycles of the tachometersinusoidal output voltage. A second diode CR2 is connected in serieswith the output of amplifier 22 and polled to have its anode connecteddirectly to the amplifier output. Thus, on negative half cycles of thetachometer output voltage, an inverted or positive voltage at the outputof amplifier 22 produces a current which is passed by diode CR2. Afeedback resistor R3 is connected between the cathode of diode CR2 andthe inverting input of amplifier 22. The ratio of the resistances ofinput resistor R2 and feedback resistor R3 determines the gain of theamplifier 22.

The full wave rectified output from operational amplifier 22 is suppliedto an integrator. More particularly, the cathode of diode CR2 isconnected to input resistor R5 of a second operational amplifier 24.Input resistor R5 is connected to the inverting input of amplifier 24.The noninverting input of amplifier 24 is connected directly to circuitground. The negative reference voltage is supplied via potentiometer R70and precision resistor R4 to the inverting input of amplifier 24.Amplifier 24 has a feedback circuit comprising a resistor R6, acapacitor C1 and a diode CR7 each connected between the output of theamplifier and its inverting input. Diode CR7 is poled to allow the flowof current from the output of the amplifier to its inverting input oncethe threshold voltage across the diode is exceeded. Thus, diode CR7serves as a clamp in the integrator feedback. The addition of diode CR7is another modification to the original circuit made by this invention.The output of amplifier 24 is connected via two series diodes CR3 andCR4 to the base of transistor Q1 to supply the base drive current to thetransistor.

Briefly summarizing, the tachometer voltage is input to amplifier 24through resistor R1. On positive half cycles of the tachometer voltage,amplifier 22 is inactive, clamped by diode CR1. On negative half cycles,the tachometer voltage is amplified in amplifier 22 by two and summed,in integrator amplifier 24, with the current from resistor R1.Therefore, on positive half cycles I_(IN) =V_(TACH) /R1 and on negativehalf cycles I_(IN) =(-V_(TACH) /R1)+(2V_(TACH) /R5)=V_(TACH) /R1, whereR1=R5.

During normal operation, the rectified tachometer output fromoperational amplifier 22 is summed with the negative reference voltagewhich is calibrated for the correct speed of the motor 10. At steadystate, a positive output voltage from operational amplifier 24 isapplied to the base of NPN transistor Q1 to provide the required drivecurrent to motor 10. The rectified tachometer output voltage fromamplifier 22 just balances the negative reference voltage and thenegative feedback voltage from feedback capacitor C1. Should the motor10 speed up producing a more positive output from amplifier 22, theinverted output of amplifier 24 would become more negative reducing thedrive current to the motor. On the other hand, should the motor 10 slowdown producing a more negative output from amplifier 22, the output ofamplifier 24 becomes more positive causing the drive current to motor 10to increase bringing its speed up to design speed.

According to the invention, in the event that motor 10 stalls due, forexample, to a high impedance contamination buildup between thecommutator and brushes, the full wave rectified output voltage fromamplifier 22 goes to zero. As a result, the negative reference voltagebecomes the only input to integrating amplifier 24. This produces apositive ramping voltage at the output of amplifier 24 which is limitedby the clamping action of diode CR7. The drive voltage on the base ofNPN transistor Q1 becomes quite high driving the transistor to a full onor conducting condition. The effective short circuit causes almost thefull +74 volt line voltage to be impressed across motor 10. This isapproximately fifteen times the normal operating voltage and is twicethe rated voltage of the motor. The emitter resistance R9 limits themotor current to approximately 200 mA. This "jolt" of voltage iseffective to restart the motor and burn off any high impedancecontamination that may have developed on the commutator. Once restarted,the feedback speed control provided by the rectified tachometer outputvoltage takes over, quickly reducing the voltage across the motor 10 andreducing the drive current through transistor Q1 to steady state.

From the foregoing, it will be appreciated that the inventionaccomplishes its objectives by providing an inexpensive solution to aunique problem. That problem manifested itself in the specificapplication of very slow running permanent magnet d.c. motors to thecapstan drive of railroad locomotive data recorders. Because of thequick response time of the modified drive circuit, the condition of astalled motor is quickly detected and the motor restarted with little orno loss of data. The solution according to the invention, however, isnot limited to data recorders of the type described. Rather, theinvention may find other applications where very slow running permanentmagnet d.c. motors are used such as in various instrumentation andcontrol systems. Therefore, those skilled in the art will understandthat while the invention has been described in terms of a specific,preferred embodiment, the invention may be practiced in other anddifferent environments and with modification within the spirit and scopeof the appended claims.

Having thus described my invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. An anti-stall motor drivecircuit for a very slow running permanent magnet d.c. motor comprising:adrive transistor having collector, base and emitter electrodes, saidmotor being connected to the collector of said drive transistor and acurrent limiting resistor being connected to the emitter of said drivetransistor; means for detecting a stalled condition of said motor; andmeans responsive to said detecting means for momentarily increasing abase drive voltage to said transistor to momentarily increase thevoltage across said motor an order of magnitude greater than normalopertating voltage and exceeding a rated voltage of the motor toovercome the stalled condition.
 2. The anti-stall motor drive circuitrecited in claim 1 wherein said detecting means comprises:a tachometerdriven by said motor, said tachometer generating a sinusoidal outputvoltage; and rectifying means connected to receive the sinusoidal outputvoltage of said tachometer and providing a rectified output voltageproportional to the speed of said motor, said rectified output voltagegoing to zero when there is a stalled condition of said motor.
 3. Theanti-stall motor drive circuit, recited in claim 2 wherein said meansresponsive to said detecting means comprises:an integrator including anoperational amplifier having an inverting input and an output, afeedback capacitor connected between said inverting input and saidoutput, and a summing junction connected to said inverting input; afirst input resistor connecting the output of said rectifying means tosaid summing junction; a second input resistor connecting saidtachometer to said summing junction; a clamping diode connected betweensaid inverting input and said output of said operational amplifier tolimit the output voltage of said integrator; and means for connectingthe output of said integrator to the base of said transistor.
 4. Theanti-stall motor drive circuit recited in claim 3 wherein said rectifiermeans comprises:a second operational amplifier having an inverting inputand an output, said inverting input being connected to a second summingjunction; a first diode connected to pass a positive current from saidsumming junction to the output of said second operational amplifier anda second diode connected to pass a positive current from said secondoperational amplifier output to said integrator; a third input resistorfor connecting said tachometer to said second summing junction; and afeedback resistor connected between said second diode and said secondsumming junction.